Microwave Spectrum and Hindered Pseudorotation of Tetrahydrofuran

The microwave spectrum of the tetrahydrofuran (C4H8O) molecule in the ground state and eight excited states of hindered pseudorotation has been studied. A strong perturbation of rotational spectra has been found for three pairs of pseudorotational states, which is due to the vibrational-rotational interaction. To analyze the nonrigid spectra of these states we used the double resonance technique. Two hundred and sixty nine rotational and vibrational-rotational transitions corresponding to the μa and μc components of the dipole moment have been identified. The rotational constants have been determined along with the quartic constants of centrifugal distortion and the spectroscopic parameters of the interaction between the overall rotation and hindered pseudorotation. The splittings of three pairs of quasidegenerate vibrational levels have been calculated: Δ01 = 21,308.17 MHz, Δ23 = 61,205.28 MHz, Δ56 = 68,183 MHz. The potential function of hindered pseudorotation V(ϕ)=- 7.84(1-cos2ϕ)/2+36.10(1-cos4ϕ)/2 (cm–1) was found from the splittings. It is concluded that the molecule has a twisted conformation (C2 symmetry) in the states ν =0 and ν =1 of hindered pseudorotation and a bent conformation (Cs symmetry) in the states ν = 2 and ν = 3. The component of the dipole moment of the transition ‹ν = 2 |μc | ν = 3› = 0.57± 0.01D was determined from the Stark effect of the rotational transitions in the ν = 2, ν = 3 states.